The disclosure relates generally to hydrous clay containing green monoliths and more specifically to hydrous clay containing green monoliths that can exhibit improved thermal expansion characteristics.
In the formation of ceramic bodies, e.g., silicon carbide, cordierite, mullite, alumina, or aluminum titanate bodies, green monoliths are typically fired at high temperatures for an extended period of time. During firing, the green bodies experience one or more phase changing events. For example, during the firing of hydrous kaolin containing green monoliths, the bodies interact with magnesia containing minerals, i.e. talc (3MgO4SiO2H2O), brucite (Mg(OH)2), magnesite MgCO3, to form cordierite (2MgO2Al2O35SiO2). During a time (and temperature) in the firing cycle that corresponds to a kaolin dehydroxylation and sintering event, a preferred orientation and grain size coursing (domains) develops, which can correlate to a given coefficient of thermal expansion (CTE) in the fired ceramic body.
Various methods have been employed to control the CTE of such hydrous clay containing green monoliths. One method, such as that described in U.S. Pat. No. 8,058,198 B2 involves controlling the amount of hydrous kaolin and the kaolin crystallinity (as measure by the XRD Hinckely index method) in the batch composition. However, reducing the amount of hydrous clay can lead to certain potential disadvantages, such as creating a less plastic batch material that is more difficult to extrude and/or undesirable changes with respect to the properties of the fired ceramic body. In addition, the crystallinity, as measured by XRD Hinckley index method, describes the order/disorder of the kaolin (i.e. it does not differentiate crystallographic directions).
Other methods have involved characterizing the clay morphology using an analytical technique, correlating such characterization with a desired characteristic, and then selecting clay that will help enable the resulting composition to exhibit the desired characteristic. For example, U.S. Pat. No. 7,481,962, discloses an analytical technique for determining a clay “cleavage index” and then setting a limit on that index for clay used in a composite batch composition.